1. Field of the Invention
The present invention relates to a projection lens for a pico-projector, and more particularly, to a projection lens for a pico-projector in which a plurality of plastic lenses and a single glass lens are arranged, to minimize a change in focal length due to the heat generated inside the pico-projector.
2. Description of the Related Art
As a display device developed and commercialized as a big screen and high resolution display device, a projector has been rising as one of the most important issues. A projector displays an image by projecting the image formed in a display element by using light emitted by a light source.
A very small projector has been developed in the name of a pico-projector. A pico-projector is installed inside or outside of a mobile device, such as a mobile phone, a notebook or a PMP, among others, or a fixed device, such as a desktop computer.
In designing lenses for pico-projectors, it is important to manufacture light lenses at low cost, to satisfy the requirements for less distortion and lateral chromatic aberration and a relatively long back focal length (BFL).
For example, FIG. 1 illustrates a conventional lens unit 100 for a mini projector, the full length of which is about 60 mm. As illustrated in FIG. 1, the lens unit 100 for the mini projector comprises a lens array 10 consisting of seven lenses, a prism 20 for color composition, a cover glass 30 and an image panel 40. The length from a 1st lens 11 to a 7th lens 17 of the lens array 10 is the full length, and the length from the 7th lens 17 (the lens positioned farthest) to the image pane 40 is the back focal length (BFL).
In the case where a plastic lens array is used to reduce the manufacturing cost of a pico-projector and to decrease the weight thereof, the focal length is changed due to the heat generated inside the pico-projector.